Insight User Guide: Natural ground subsidence

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Shrink-swell clays

The potential hazard presented by soils that absorb water when wet (making them swell), and lose water as they dry (making them shrink). This shrink-swell behaviour is controlled by the type and amount of clay in the soil, and by seasonal changes in the soil moisture content (related to rainfall and local drainage).

A - Negligible	Ground conditions predominantly non-plastic.
B - Very low	Ground conditions predominantly low plasticity.
C - Low	Ground conditions predominantly medium plasticity.
D - Moderate	Ground conditions predominantly high plasticity.
E - High	Ground conditions predominantly very high plasticity.

Data source: British Geological Survey
Data update schedule: Quarterly

Running sands

The potential hazard presented by rocks that can contain loosely-packed sandy layers that can become fluidised by water flowing through them. Such sands can ‘run’, removing support from overlying buildings and causing potential damage.

A - Negligible	Running sand conditions are not thought to occur whatever the position of the water table. No identified constraints on land use due to running conditions
B - Very low	Running sand conditions are unlikely. No identified constraints on land use due to running conditions unless water table rises rapidly
C - Low	Running sand conditions may be present. Constraints may apply to land uses involving excavation or the addition or removal of water.
D - Moderate	Running sand conditions are probably present. Constraints may apply to land uses involving excavation or the addition or removal of water.
E - High	Running sand conditions are almost certainly present. Constraints will apply to land uses involving excavation or the addition or removal of water.

Data source: British Geological Survey
Data update schedule: Quarterly

Compressible deposits

The potential hazards presented by natural deposits that could compress when a load (such as a building) is placed on them or they become saturated with water.

A - Negligible	Compressible strata are not thought to occur.
B - Very low	Compressibility and uneven settlement problems are not likely to be significant on the site for most land uses.
C - Low	Compressibility and uneven settlement potential may be present. Land use should consider specifically the compressibility and variability of the site.
D - Moderate	Compressibility and uneven settlement hazards are probably present. Land use should consider specifically the compressibility and variability of the site.
E - High	Highly compressible strata present. Significant constraint on land use depending on thickness.

Data source: British Geological Survey
Data update schedule: Quarterly

Collapsible deposits

The potential hazards presented by natural deposits that could collapse when a load (such as a building) is placed on them or they become saturated with water.

A - Negligible	Deposits with potential to collapse when loaded and saturated are believed not to be present.
B - Very low	Deposits with potential to collapse when loaded and saturated are unlikely to be present.
C - Low	Deposits with potential to collapse when loaded and saturated are possibly present in places.
D - Moderate	Deposits with potential to collapse when loaded and saturated are probably present in places.
E - High	Deposits with potential to collapse when loaded and saturated are present.

Data source: British Geological Survey
Data update schedule: Quarterly

Landslides

The potential for landsliding (slope instability) to be a hazard assessed using 1:50,000 scale digital maps of superficial and bedrock deposits, combined with information from the BGS National Landslide Database and scientific and engineering reports.

A - Negligible	Slope instability problems are not thought to occur but consideration to potential problems of adjacent areas impacting on the site should always be considered.
B - Very low	Slope instability problems are not likely to occur but consideration to potential problems of adjacent areas impacting on the site should always be considered.
C - Low	Slope instability problems may be present or anticipated. Site investigation should consider specifically the slope stability of the site.
D - Moderate	Slope instability problems are probably present or have occurred in the past. Land use should consider specifically the stability of the site.
E - High	Slope instability problems almost certainly present and may be active. Significant constraint on land use.

Data source: British Geological Survey
Data update schedule: Quarterly

Ground dissolution of soluble rocks

The potential hazard presented by ground dissolution, which occurs when water passing through soluble rocks produces underground cavities and cave systems. These cavities reduce support to the ground above and can cause localised collapse of the overlying rocks and deposits.

A - Negligible	Soluble rocks are present within the ground. Very few dissolution features are likely to be present. Potential for difficult ground conditions or localised subsidence are at a level where they need not be considered.
B - Very low	Soluble rocks are present within the ground. Few dissolution features are likely to be present. Potential for difficult ground conditions or localised subsidence are at a level where they need not be considered except in exceptional circumstances.
C - Low	Soluble rocks are present within the ground. Some dissolution features may be present. Potential for difficult ground conditions are at a level where they may be considered, localised subsidence need not be considered except in exceptional circumstances.
D - Moderate	Soluble rocks are present within the ground. Some dissolution features may be present. Potential for difficult ground conditions are at a level where they may be considered, localised subsidence need not be considered except in exceptional circumstances.
E - High	Soluble rocks are present within the ground. Numerous dissolution features may be present. Potential for difficult ground conditions should be investigated. Potential for localised subsidence is at a level where it should be considered.

Data source: British Geological Survey
Data update schedule: Quarterly